A non-suppurative migratory polyarthropathy with gross and microscopic features suggestive of certain findings in human rheumatoid arthritis has been found to occur following infection of rats with Salmonella enteritidis. Initial observations suggest mediation of the joint disease by an immune process rather than local sepsis. A detailed investigation of this Salmonella-associated arthritis (SAA) is proposed to determine whether this readily inducible joint disease does indeed constitute a useful model of human rheumatoid arthritis. Methodology will involve a correlated study employing standard methods of pathology and microbiology to evaluate the role of infection in the development of SAA. This will include culture of tissues and organs and the use of antibiotics. Attempts will be made to elicit the disease with purified antigens. Immunological considerations will include a study of serum protein changes and the detection of cell and tissue-bound antigens by means of immunofluorescence. Passive transfer of the joint lesions will be attempted with lymphoid cells and serum. The role of immune systems will also be assessed in hosts that are immunologically altered by thymus deprivation, immunosuppressive drugs, ionizing radiation and antilymphocytic sera. The results will be compared with documented alterations of the corresponding aspects of human rheumatoid arthritis.